1. Field of the Invention
The present invention relates to a method for surface modification by which a crystalline state of a surface of a material is modified such as a semiconductor material.
2. Description of the Related Art
GaN-LEDs (light-emitting diodes) have been increasingly used in fabrication of backlights, illuminating devices, or the like of liquid crystal panels. Accordingly, technologies for outputting GaN-LEDs with a high efficiency have been increasingly developed.
Outputting GaN-LEDs with a high efficiency may mainly involve two methods, one of which is a method of increasing internal quantum efficiency and the other of which is a method of increasing extraction efficiency of light actually emitted. With regard to the former method, internal quantum efficiency has been increased to a certain extent by reducing a defect density in a GaN crystal; whereas the latter method, extraction efficiency of light has been increased by improving a structure of a light-emitting edge or combining a GaN crystal with a resin lens (also serving as a sealing).
In improvement of a structure of a light-emitting edge, K. Orita et al., “High-Extra-Efficiency Blue Light-Emitting Diode Using Extended-Pitch Photonic Crystal”, Japanese Journal of Applied Physics, Vol. 43, No. 8B, (2004), pp. 5809-5813 has reported a case where a two-dimensional periodic structure (a photonic crystal in a broad sense) is provided on an entire or portion of a light-emitting edge of LED to increase extraction efficiency of light, for example. According to the aforementioned Japanese Journal of Applied Physics, a two-dimensional structure is formed on one surface of a semiconductor light-emitting element as shown in a schematic sectional view of FIG. 1. An n-type GaN clad layer 52, an InGaN active layer 53, and a p-type GaN clad layer 54 are formed on a substrate 51 formed of sapphire, and a two-dimensional structure 60 is formed only on the p-type GaN clad layer 54 by a lithographic technology. A transparent conductive layer 55 is formed on the two-dimensional structure 60 and is connected to an electrode 56 and a wiring 57. Extraction power having such a configuration is about 1.5 times that of a configuration in which a two-dimensional structure is not provided.
When such a periodic structure is formed in an LED, the structure may be formed in a portion extremely close to a light source and may be formed in any shape. Therefore, emission of light from an active layer may be more efficiently controlled, and high extraction efficiency may be realized. However, when the periodic structure is formed by a typical semiconductor lithographic technology currently used, a simple method with which a high cost performance is achieved may be desired, since a high-precision photomask may be necessary and formation steps are complicated, for example.
In contrast, there has been proposed a more simple method of forming various structures including a periodic structure by laser drawing or interference exposure. In this method, a photomask may not have to be used and an extremely simple processing step may be employed. However, a laser processing process of the related art including melting may involve such drawbacks as a low shape accuracy and debris.
In recent years, a processing process using a femtosecond laser has been attracted attention as a laser process and shape processing may be expected to be performed with higher accuracy. In the processing process using a femtosecond laser, a bond may be directly broken by a multiphoton absorption process, and the absorption process is finished before thermal vibration of atoms or molecules starts, thereby achieving a non-thermal processing process. In this case, accuracy degradation in thermal processing may be eliminated, and debris may also be treated in a relatively simple cleaning process in which no melting marks are formed. Thus, as Japanese Unexamined Patent Application Publication No. 2003-25085, for example, discloses that not only may more accurate processed shapes be expected, but all the processing steps may also be simplified.